Epidermal growth factor receptor gene (EGFR) is a transmembrane tyrosine-kinase receptor that belongs to the epidermal growth factor family of receptors (ErbB family), which includes four closely related receptor tyrosine kinases: EGFR (ErbB-1), HER2/c-neu (ErbB-2), Her 3 (ErbB-3) and Her 4 (ErbB-4). Upon ligand binding, EGFR activates intracellular signaling pathways, mainly the RAS-RAF-MEK-ERK cascade and the PI3KAkt pathway, that regulate key oncogenic events such as apoptosis, cell growth, angiogenensis and metastasis. Aberrant activation or overexpression of EGFR has been reported in several types of cancer (i.e. Mendelsohn J, Baselga J et al., “Epidermal growth factor receptor targeting in cancer”. Semin Oncol —2006, Vol. 33, pp.: 369-38). Mutations in EGFR gene have been described in lung cancer. Examples of such mutation are disclosed for instance in the document of Lynch T J et al., “Activating mutations in the epidermal growth factor receptor underlying responsiveness of non-small-cell lung cancer to gefitinib”, N Engl J Med—2004, Vol. 350, pp: 2129-2139; or in Paez J G et al., “EGFR mutations in lung cancer: correlation with clinical response to gefitinib therapy”, Science—2004, Vol. 304, pp.: 1497-500; or in Pao W et al., “EGF receptor gene mutations are common in lung cancers from “never smokers” and are associated with sensitivity of tumors to gefitinib and erlotinib”, Proc Natl Acad Sci USA—2004, Vol. 101, pp.: 13306-13311.).
Metastasic colorectal cancer (mCRC) is the second leading cause of death from cancer in the Western Countries world.
A therapy based on monoclonal antibodies (moAbs), e.g. cetuximab and panitumumab, which are directed against EGFR, provides significant survival benefit to patients with mCRC and are now standard components of therapy regimens for these patients, i.e. either alone or in combination with other antineoplasic drug(s). One of these moAbs, cetuximab (Erbitux) is also indicated for the treatment of patients with squamous cell carcinoma of the head and neck, also named head and neck cancer, in combination with platinum-based chemotherapy.
The moAbs bind to foreign antigens expressed on cancer cells. Once bound, the cancer cells are marked for destruction by the patient's immune system. In addition to targeting cancer cells, moAbs can be designed to act on other cell types and molecules necessary for tumor growth. For example, antibodies can neutralize growth factors and thereby inhibit tumor expansion. It is possible to create a moAb specific to almost any extracellular/cell surface target (such as cancer cells). In summary, moAbs can be used to destroy malignant tumor cells and prevent tumor growth by blocking specific cell receptors. Therapeutic moAbs cetuximab and panitumumab bind to EGFR and prevent the activation of intracellular signaling pathways driven by EGFR (i.e., the RAS-RAF-MEK-ERK cascade and PI3K-akt pathway).
Unfortunately, not all patients with mCRC respond to a therapy regimen comprising moAbs. The lack of response of a patient with mCRC to such a treatment could be primary, i.e. since the beginning of anti-EGFR moAb treatment; known as primary resistance. Moreover, all mCRC patients that initially respond to anti-EGFR moAbs invariably develop secondary resistance, i.e. acquired resistance to anti-EGFR moAb. In both cases, the result is treatment failure. The mechanisms that contribute to the acquisition of such treatment resistance in mCRC patients is still not fully known. The same resistance to anti-EGFR moAb therapy (primary or secondary) is observed in patients with head and neck cancer.
KRAS (also known as V-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog) is an EGFR downstream effector, and a marker of primary resistance to anti-EGFR moAbs. KRAS has a significant impact on the optimization of treatment of mCRC patients. Forty percent of colorectal tumors harbour a mutation in the KRAS gene and these patients do not benefit from anti-EGFR moAbs. In current clinical practice all mCRC patients who are being considered for anti-EGFR moAb therapy should undergo KRAS testing, and patients should be excluded from cetuximab or panitumumab therapy if a KRAS mutation is detected.
Nevertheless, a fraction of mCRC patients with wild-type KRAS tumors still do not benefit from anti-EGFR moAbs. The response rate to anti-EGFR moAbs in wild-type KRAS patients is approximately 60% when combined with chemotherapy and less than 20% when administered alone in chemotherapy-refractory patients, as derived from Amado et al., “Wild-type KRAS is required for panitumumab efficacy in patients with metastasic colorectal cancer”, J. Clin Oncol —2008, Vol. 28, pp.: 1626-1634.
Activating mutations of other EGFR downstream genes such as BRAF (serine/threonine-protein kinase B-Raf) and PI3K (phosphatidylinositol 3-kinase), as well as loss of expression of PTEN (phosphatase and tensin homolog), and alterations in other EGFR regulatory proteins are being evaluated as potential candidates for response to anti-EGFR therapy with inconclusive results so far. Information regarding the association between the mutations in theses genes and the response to anti-EGFR therapy can be derived from the documents of De Roock et al., “Effects of KRAS, BRAF, NRAS, and PIK3CA mutations on the efficacy of cetuximab plus chemotherapy in chemotherapy-refractory metastasic colorectal cancer: a retrospective consortium analysis”, Lancet Oncol—2010, Vol. 11, pp.: 753-762; or in the document of Loupakis et al., “PTEN expression and KRAS mutations on primary tumors and metastases in the prediction of benefit of cetuximab plus irinotecan for patients with metastasic colorectal cancer”, J Clin Oncol —2009, Vol. 27, pp.: 2622-2629.
The studies carried out so far to elucidate a potential role of the EGFR as a marker of response to anti-EGFR moAb are inconclusive. EGFR protein expression, as detected by immunohistochemistry, is not a reliable predictive marker of response to anti-EGFR moAbs. However, there is increasing evidence supporting EGFR gene copy number as a potential biomarker of response to anti-EGFR moAbs. Regarding the association of nucleotide changes in the EGFR gene with response to anti-EGFR moAbs-based therapy, the state of the art, in particular Gonçalves et al. In “A polymorphism of EGFR extracellular domain is associated with progression free-survival in metastasic colorectal cancer patients receiving cetuximab-based treatment”, BMC Cancer—2008, Vol. 8, pp.: 169, describes a polymorphism in the extracellular portion of the EGFR gene, resulting in the amino acid substitution R521K associated with cetuximab benefit in mCRC patients. The polymorphism or single nucleotide polymorphism is the one identified as the variation CM942312 from January 2011, retrievable from the database Ensembl (www.ensembl.org). It corresponds to the codon change at position 521 AGG-AAG in the mRNA sequence identified as NM_005228 version 3, available on 26.06.2011 from GenBank.
Additionally, also document WO2008/88860 discloses that patients with metastasic or non-metastasic gastrointestinal neoplasm or malignant tumour having the polymorphism R497K in the EGFR gene are likely to show responsiveness to single agent anti-EGFR moAb-based therapy (e.g. cetuximab or panitumumab). This mutation is the same disclosed by Gonçalves et al. (supra), but being identified with the ancient designation. Finally, document WO2005085473 discloses the association of 12 polymorphisms in the regulatory region of the EGFR gene, which induce over-expression of the EGFR protein, with decreased efficacy of an EGFR-targeting therapeutic agent for the treatment of cancer in a patient.
In summary, the results showed in the documents comprised in the state of the art are not only inconclusive, but also do not fully clarify the fraction of mCRC patients with wild-type KRAS tumors who still do not benefit from anti-EGFR mAb-based therapy.
In view of the above, it is therefore necessary to identify additional predictive biomarkers of resistance to anti-EGFR moAb therapy in patients with mCRC.